As a method for producing high-purity polycrystalline silicon used as a raw material for monocrystalline silicon for production of a semiconductor, a Siemens method and a fluidized bed reactor method are known. The Siemens method is a method in which a raw material gas containing chlorosilane is contacted with a heated silicon core, and polycrystalline silicon is grown on the surface of the silicon core by a CVD (Chemical Vapor Deposition) method. The fluidized bed reactor method is a method in which monosilane or trichlorosilane as a raw material is fed and vapor-deposited in a fluidizing gas to obtain granular polysilicon.
In a step of producing such polycrystalline silicon, a large amount of heat needs to be supplied in order to keep silicon being deposited. From the viewpoint of reduction in cost, it is preferable to recover and reuse the heat supplied in the production step. The polycrystalline silicon for production of a semiconductor is required to have extremely high purity. For this reason, it is insufficient only to realize a production process having a highly efficient heat recovery and contamination with impurities in the polycrystalline silicon during the production step needs to be prevented as much as possible.
For example, U.S. Pat. No. 4,724,160 (Patent Literature 1) discloses a system in which heat exchange is performed between a coolant used to cool a metal reactor vessel (reactor) and a steam generator to generate steam and heat is reused using the steam as a heating source (so-called, steam recovery). If such a method using heat exchange between the coolant and the steam generator is used, the temperature of the inner wall of the reactor vessel has to be a high temperature to some extent.
Patent Literature 1 exemplifies polyorganosiloxane as the coolant. In the case where polyorganosiloxane is used as the coolant, the boundary film heat transfer coefficient is relatively small due to thermal properties such as specific heat and heat conductivity of polyorganosiloxane. When polycrystalline silicon grows to increase the diameter of the polycrystalline silicon, the temperature of the inner wall surface of the steel reactor vessel reaches 400° C. or more.
If the temperature of the inner wall surface of the steel reactor vessel is not less than 400° C., the inner wall surface of the reactor vessel contacting a process gas obtained by diluting a silicon raw material gas such as trichlorosilane with hydrogen gas is gradually corroded. For this, with chemical components in steel that constitutes the inner wall surface, impurity elements such as phosphorus, arsenic, boron, and aluminum contained in the steel are also discharged into the reaction atmosphere. These impurity elements undesirably act as a dopant in the polycrystalline silicon to give an influence to resistivity, leading to drastic reduction in quality.
In consideration of such problems, Japanese Patent Laid-Open No. 8-259211 (Patent Literature 2) discloses a technique for obtaining deposited high-purity silicon by depositing silicon within a reactor formed with a material that hardly outgases.
Specifically, based on a knowledge that a heat-resistant alloy containing not less than 28% by weight of nickel hardly outgases at a temperature of not more than 600° C., the decomposition and reduction reaction of silanes are performed within a reactor vessel having an inner wall comprising a heat-resistant alloy containing not less than 28% by weight of nickel, thereby to further increase the purity of polycrystalline silicon to be obtained. Examples of the above-described “heat-resistant alloy containing not less than 28% by weight of nickel” include Incoloy 800, Inconel 600, Inconel 601, Incoloy 825, Incoloy 801, Hastelloy B, and Hastelloy C.